This study will be conducted among a large multiethnic cohort (N = 11,432) from the nationwide Women's Health Initiative (WHI), which includes an observational study and three clinic trials. The age range of this cohort is between 50-79 years at the baseline, and it has multiple minority groups: 1583 black, 739 Hispanic and 149 Native American women. The maximal follow-up time of this cohort will be 9 years by 2005. Dual-energy x-ray absorptiometry (DXA) is used to measure bone mineral density (BMD) and body composition. The randomized clinical trials and longitudinal nature of the WHI study provide a unique opportunity to investigate: 1) treatment effects of hormone replacement therapy (HRT) and calcium and vitamin D supplementation on hip structural geometry; 2) longitudinal changes in skeletal muscle mass as a factor in hip fragility; and 3) ethnic differences of mean and rates of changes in hip geometry and muscle mass. Special computer software will be used for analyzing hip scans by dual-energy x-ray absorptiometry (DXA). Cross-sectional area, subperiosteal width, estimated endocortical diameter, estimated mean cortical thickness, buckling ratio and section modulus at the femoral neck, at the intertrochanteric and the femoral shaft regions will be assessed. MRI scans will be used as references to calibrate total, appendicular and leg skeletal muscle measurements from DXA subregion analyses. Prevalence rates of sarcopenia (low muscle mass) among each age and ethnic group will be studied. Random Effects Models will be used to analyze the longitudinal data. This proposed study is not funded by the WHI program. Recourses that the WHI will provide include DXA scans, fall and fracture data, and information on covariates. Since the majority of data collection work has been or will be done by the WHI, we will be able to cost effectively test multiple important scientific hypotheses in this study. The novel approaches in this ancillary study will enhance scientific contributions of the WHI program. The significance of the proposed study is that it may demonstrate the utility of bone structural analysis in addition to bone mass measurements for understanding ethnic differences in fracture risk and/or for assessing the effect of pharmacologic therapy (i.e. HRT) on bone health. Furthermore, if the muscle variables are found to be a strong determinant of bone structure in the proximal femur and the risk of fall, then it may be important to develop interventions to increase muscle mass in this region to prevent hip fracture.